SIPPING Working Group J. Hautakorpi, Ed.
Internet-Draft G. Camarillo
Intended status: Informational Ericsson
Expires: August 22, 2010 R. Penfield
Acme Packet
A. Hawrylyshen
Ditech Networks Inc.
M. Bhatia
3CLogic
February 18, 2010
Requirements from Session Initiation Protocol (SIP) Session BorderControl (SBC) Deploymentsdraft-ietf-sipping-sbc-funcs-09.txt
Abstract
This document describes functions implemented in Session Initiation
Protocol (SIP) intermediaries known as Session Border Controllers
(SBCs). The goal of this document is to describe the commonly
provided functions of SBCs. A special focus is given to those
practices that are viewed to be in conflict with SIP architectural
principles. This document also explores the underlying requirements
of network operators that have led to the use of these functions and
practices in order to identify protocol requirements and determine
whether those requirements are satisfied by existing specifications
or additional standards work is required.
Status of this Memo
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Internet-Draft Requirements from SIP SBC Deployments February 2010http://www.ietf.org/shadow.html.
This Internet-Draft will expire on August 22, 2010.
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Internet-Draft Requirements from SIP SBC Deployments February 20101. Introduction
In the past few years there has been a rapid adoption of the Session
Initiation Protocol (SIP) [1] and deployment of SIP-based
communications networks. This has often outpaced the development and
implementation of protocol specifications to meet network operator
requirements. This has led to the development of proprietary
solutions. Often, these proprietary solutions are implemented in
network intermediaries known in the marketplace as Session Border
Controllers (SBCs) because they typically are deployed at the border
between two networks. The reason for this is that network policies
are typically enforced at the edge of the network.
Even though many SBCs currently behave badly in a sense that they
break end-to-end security and impact feature negotiations, there is
clearly a market for them. Network operators need many of the
features current SBCs provide and often there are no standard
mechanisms available to provide them.
The purpose of this document is to describe functions implemented in
SBCs. A special focus is given to those practices that are
conflicting with SIP architectural principles in some way. The
document also explores the underlying requirements of network
operators that have led to the use of these functions and practices
in order to identify protocol requirements and determine whether
those requirements are satisfied by existing specifications or
additional standards work is required.
2. Background on SBCs
The term SBC is relatively non-specific, since it is not standardized
or defined anywhere. Nodes that may be referred to as SBCs but do
not implement SIP are outside the scope of this document.
SBCs usually sit between two service provider networks in a peering
environment, or between an access network and a backbone network to
provide service to residential and/or enterprise customers. They
provide a variety of functions to enable or enhance session-based
multi-media services (e.g., Voice over IP). These functions include:
a) perimeter defense (access control, topology hiding, and denial of
service prevention and detection); b) functionality not available in
the endpoints (NAT traversal, protocol interworking or repair); and
c) traffic management (media monitoring and QoS). Some of these
functions may also get integrated into other SIP elements (like pre-
paid platforms, 3GPP P-CSCF [6], 3GPP I-CSCF, etc).
SIP-based SBCs typically handle both signaling and media and can
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implement behavior which is equivalent to a "privacy service" (as
described in[2]) performing both Header Privacy and Session Privacy).
SBCs often modify certain SIP headers and message bodies that proxies
are not allowed to modify. Consequently, they are, by definition,
B2BUAs (Back-to-Back User Agents). The transparency of these B2BUAs
varies depending on the functions they perform. For example, some
SBCs modify the session description carried in the message and insert
a Record-Route entry. Other SBCs replace the value of the Contact
header field with the SBCs address, and generate a new Call-ID and
new To and From tags.
+-----------------+
| SBC |
[signaling] | +-----------+ |
<------------|->| signaling |<-|---------->
outer | +-----------+ | inner
network | | | network
| +-----------+ |
<------------|->| media |<-|---------->
[media] | +-----------+ |
+-----------------+
Figure 1: SBC architecture
Figure 1 shows the logical architecture of an SBC, which includes a
signaling and a media component. In this document, the terms outer
and inner network are used for describing these two networks. An SBC
is logically associated to the inner network, and it typically
provides functions such as controlling and protecting access to the
inner network from the outer network. The SBC itself is configured
and managed by the organization operating the inner network.
In some scenarios SBCs operate with users' (implicit or explicit)
consent while in others they operate without users' consent (this
latter case can potentially cause problems). For example, if an SBC
in the same administrative domain as a set of enterprise users
performs topology hiding (see Section 3.1), the enterprise users can
choose to route their SIP messages through it. If they choose to
route through the SBC, then the SBC can be seen as having the users'
implicit consent. Another example is a scenario where a service
provider has broken gateways and it deploys an SBC in front of them
for protocol repair reasons (see Section 3.6). Users can choose to
configure the SBC as their gateway and, so, the SBC can be seen as
having the users' implicit consent.
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Internet-Draft Requirements from SIP SBC Deployments February 20102.1. Peering Scenario
A typical peering scenario involves two network operators who
exchange traffic with each other. An example peering scenario is
illustrated in Figure 2. An originating gateway (GW-A1) in operator
A's network sends an INVITE that is routed to the SBC in operator B's
network. Then the SBC forward it to the softswitch (SS-B). The
softswitch responds with a redirect (3xx) message back to the SBC
that points to the appropriate terminating gateway (GW-B1) in
operator B's network. If operator B would not have an SBC, the
redirect message would go to the operator A's originating gateway.
After receiving the redirect message, the SBC sends the INVITE to the
terminating gateway.
Operator A . Operator B
.
. 2) INVITE
+-----+ . /--------------->+-----+
|SS-A | . / 3) 3xx (redir.) |SS-B |
+-----+ . / /---------------+-----+
. / /
+-----+ 1) INVITE +-----+--/ / +-----+
|GW-A1|---------------->| SBC |<---/ 4) INVITE |GW-B1|
+-----+ +-----+---------------------->+-----+
.
+-----+ . +-----+
|GW-A2| . |GW-B2|
+-----+ . +-----+
Figure 2: Peering with SBC
From the SBC's perspective the Operator A is the outer network, and
Operator B is the inner network. Operator B can use the SBC, for
example, to control access to its network, protect its gateways and
softswitches from unauthorized use and Denial of Service (DoS)
attacks, and monitor the signaling and media traffic. It also
simplifies network management by minimizing the number ACL (Access
Control List) entries in the gateways. The gateways do not need to
be exposed to the peer network, and they can restrict access (both
media and signaling) to the SBCs. The SBC helps ensure that only
media from sessions the SBC authorizes will reach the gateway.
2.2. Access Scenario
In an access scenario, presented in Figure 3, the SBC is placed at
the border between the access network (outer network) and the
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operator's network (inner network) to control access to the
operator's network, protect its components (media servers,
application servers, gateways, etc.) from unauthorized use and DoS
attacks, and monitor the signaling and media traffic. Also, since
the SBC is call stateful, it may provide access control functions to
prevent over-subscription of the access links. Endpoints are
configured with the SBC as their outbound proxy address. The SBC
routes requests to one or more proxies in the operator network.
Access Network Operator Network
+-----+
| UA1 |<---------\
+-----+ \
\
+-----+ \------->+-----+ +-------+
| UA2 |<-------------------->| SBC |<----->| proxy |<-- -
+-----+ /--->+-----+ +-------+
/
+-----+ +-----+ /
| UA3 +---+ NAT |<---/
+-----+ +-----+
Figure 3: Access scenario with SBC
The SBC may be hosted in the access network (e.g,. this is common
when the access network is an enterprise network), or in the operator
network (e.g., this is common when the access network is a
residential or small business network). Despite where the SBC is
hosted, it is managed by the organization maintaining the operator
network.
Some endpoints may be behind enterprise or residential NATs. In
cases where the access network is a private network, the SBC is a NAT
for all traffic. It is noteworthy that SIP traffic may have to
traverse more that one NAT. The proxy usually does authentication
and/or authorization for registrations and outbound calls. The SBC
modifies the REGISTER request so that subsequent requests to the
registered address-of-record are routed to the SBC. This is done
either with a Path header field [3], or by modifying the Contact to
point at the SBC.
The scenario presented in this section is a general one, and it
applies also to other similar settings. One example from a similar
setting is the one where an access network is the open internet, and
the operator network is the network of a SIP service provider.
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Internet-Draft Requirements from SIP SBC Deployments February 20103. Functions of SBCs
This section lists those functions that are used in SBC deployments
in current communication networks. Each subsection describes a
particular function or feature, the operators' requirements for
having it, explanation of any impact to the end-to-end SIP
architecture, and a concrete implementation example. Each section
also discusses potential concerns specific to that particular
implementation technique. Suggestions for alternative implementation
techniques that may be more architecturally compatible with SIP are
outside the scope of this document.
All the examples given in this section are simplified; only the
relevant header lines from SIP and SDP (Session Description Protocol)
[7] messages are displayed.
3.1. Topology Hiding3.1.1. General Information and Requirements
Topology hiding consists of limiting the amount of topology
information given to external parties. Operators have a requirement
for this functionality because they do not want the IP addresses of
their equipment (proxies, gateways, application servers, etc) to be
exposed to outside parties. This may be because they do not want to
expose their equipment to DoS attacks, they may use other carriers
for certain traffic and do not want their customers to be aware of
it, or they may want to hide their internal network architecture from
competitors or partners. In some environments, the operator's
customers may wish to hide the addresses of their equipment or the
SIP messages may contain private, non-routable addresses.
The most common form of topology hiding is the application of header
privacy (see Section 5.1 of [2]), which involves stripping Via and
Record-Route headers, replacing the Contact header, and even changing
Call-IDs. However, in deployments which use IP addresses instead of
domain names in headers that cannot be removed (e.g. From and To
headers), the SBC may replace these IP addresses with its own IP
address or domain name.
For a reference, there are also other ways of hiding topology
information than inserting an intermediary, like an SBC, to the
signaling path. One of the ways is the User Agent (UA) driven
privacy mechanism [8], where the UA can facilitate the conceal of
topology information.
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Internet-Draft Requirements from SIP SBC Deployments February 20103.1.2. Architectural Issues
Performing topology hiding as described above by SBCs that do not
have the users' consent presents some issues. This functionality is
based on a hop-by-hop trust model as opposed to an end-to-end trust
model. The messages are modified without the subscriber's consent
and could potentially modify or remove information about the user's
privacy, security requirements, and higher-layer applications which
are communicated end-to-end using SIP. Neither user agent in an end-
to-end call has any way to distinguish the SBC actions from a Man-In-
The-Middle (MitM) attack.
Topology hiding function does not work well with Authenticated
Identity Management [4] in scenarios where the SBC does not have any
kind of consent from the users. The Authenticated Identity
Management mechanism is based on a hash value that is calculated from
parts of From, To, Call-Id, CSeq, Date, and Contact header fields
plus from the whole message body. If the authentication service is
not provided by the SBC itself, the modification of the forementioned
header fields and the message body is in violation of [4]. Some
forms of topology hiding are in violation, because they are e.g.,
replacing the Contact header of a SIP message.
3.1.3. Example
The current way of implementing topology hiding consists of having an
SBC act as a B2BUA (Back-to-Back User Agent) and remove all traces of
topology information (e.g., Via and Record-Route entries) from
outgoing messages.
Imagine the following example scenario: The SBC
(p4.domain.example.com) receives an INVITE request from the inner
network, which in this case is an operator network. The received SIP
message is shown in Figure 4.
INVITE sip:callee@u2.domain.example.com SIP/2.0
Via: SIP/2.0/UDP p3.middle.example.com;branch=z9hG4bK48jq9w174131.1
Via: SIP/2.0/UDP p2.example.com;branch=z9hG4bK18an6i9234172.1
Via: SIP/2.0/UDP p1.example.com;branch=z9hG4bK39bn2e5239289.1
Via: SIP/2.0/UDP u1.example.com;branch=z9hG4bK92fj4u7283927.1
Contact: sip:caller@u1.example.com
Record-Route: <sip:p3.middle.example.com;lr>
Record-Route: <sip:p2.example.com;lr>
Record-Route: <sip:p1.example.com;lr>
Figure 4: INVITE Request Prior to Topology Hiding
Then the SBC performs a topology hiding function. In this scenario,
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the SBC removes and stores all existing Via and Record-Route headers,
and then inserts Via and Record-Route header fields with its own SIP
URI. After the topology hiding function, the message could appear as
shown in Figure 5.
INVITE sip:callee@u2.domain.example.com SIP/2.0
Via: SIP/2.0/UDP p4.domain.example.com;branch=z9hG4bK92es3w230129.1
Contact: sip:caller@u1.example.com
Record-Route: <sip:p4.domain.example.com;lr>
Figure 5: INVITE Request After Topology Hiding
Like a regular proxy server that inserts a Record-Route entry, the
SBC handles every single message of a given SIP dialog. If the SBC
loses state (e.g., SBC restarts for some reason), it may not be able
to route messages properly (note: some SBCs preserve the state
information also on restart). For example, if the SBC removes "Via"
entries from a request and then restarts, thus losing state, the SBC
may not be able to route responses to that request; depending on the
information that was lost when the SBC restarted.
This is only one example of topology hiding. Besides topology hiding
(i.e., information related to network elements is beeing hidden),
SBCs may also do identity hiding (i.e., information related to
identity of subscribers in beeing hidden). While performing identity
hiding, SBCs may modify Contact header field values and other header
fields containing identity information. The header fields containing
identity information is listed in Section 4.1 of [2]. Since the
publication of [2], the following header fields containing identity
information have been defined: "P-Asserted-Identity", "Referred-By",
"Identity", and "Identity-Info".
3.2. Media Traffic Management3.2.1. General Information and Requirements
Media traffic management is the function of controlling media
traffic. Network operators may require this functionality in order
to control the traffic being carried on their network on behalf of
their subscribers. Traffic management helps the creation of
different kinds of billing models (e.g., video telephony can be
priced differently to voice-only calls) and it also makes it possible
for operators to enforce the usage of selected codecs.
One of the use cases for media traffic management is the
implementation of intercept capabilities where required to support
audit or legal obligations. It is noteworthy that the legal
obligations mainly apply to operators providing voice services, and
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those operators typically have infrastructure (e.g., SIP proxies
acting as B2BUAs) for providing intercept capabilities even without
SBCs.
Since the media path is independent of the signaling path, the media
may not traverse through the operator's network unless the SBC
modifies the session description. By modifying the session
description the SBC can force the media to be sent through a media
relay which may be co-located with the SBC. This kind of traffic
management can be done, for example, to ensure a certain QoS (Quality
of Service) level, or to ensure that subscribers are using only
allowed codecs. It is noteworthy that the SBCs do not have direct
ties to routing topology and they do not, for example, change
bandwidth reservations on Traffic Engineering (TE) tunnels, nor they
have direct interaction with routing protocol.
Some operators do not want to manage the traffic, but only to monitor
it for collecting statistics and making sure that they are able to
meet any business service level agreements with their subscribers
and/or partners. The protocol techniques, from the SBC's viewpoint,
needed for monitoring media traffic are the same as for managing
media traffic.
SBCs on the media path are also capable of dealing with the "lost
BYE" issue if either endpoint dies in the middle of the session. The
SBC can detect that the media has stopped flowing and issue a BYE to
both sides to cleanup any state in other intermediate elements and
the endpoints.
One possible form of media traffic management is that SBCs terminate
media streams and SIP dialogs by generating BYE requests. This kind
of procedure can take place, for example, in a situation where the
subscriber runs out of credits. Media management is needed to ensure
that the subscriber cannot just ignore the BYE request generated by
the SBC and continue their media sessions.
3.2.2. Architectural Issues
Implementing traffic management in this manner requires the SBC to
access and modify the session descriptions (i.e., offers and answers)
exchanged between the user-agents. Consequently, this approach does
not work if user-agents encrypt or integrity-protect their message
bodies end-to-end. Again, messages are modified without subscriber
consent, and user-agents do not have any way to distinguish the SBC
actions from an attack by a MitM. Furthermore, this is in violation
of Authenticated Identity Management [4], see Section 3.1.2.
The insertion of a media relay can prevent "non-media" uses of media
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path, for example media path key agreement. Sometimes this type of
prevention is intentional, but it is not always necessary. For
example, if an SBC is used just for enabling media monitoring, but
not for interception.
There are some possible issues related to the media relaying. If the
media relaying is not done in a correct manner, it may break
functions like Explicit Congestion Notification (ECN) and Path MTU
Discovery (PMTUD), for example. The media relays easily break such
IP and transport layer functionalities that rely on the correct
handling of the protocol fields. Some especially sensitive field
are, for example, ECN and Type of Service (TOS) fields, and Don't
Fragment (DF) bit.
Media traffic management function also hinders innovations. The
reason for the hinderance is that in many cases SBCs need to be able
to support new ways of communicating (e.g., extensions to the SDP
protocol) before new services can be taken into use, and that slows
down the adoption of innovations.
If an SBC directs many media streams through a central point in the
network, it is likely to cause a significant amount of additional
traffic to a path to that central point. This might create possible
bottleneck in the path.
In this application, the SBC may originate messages that the user may
not be able to authenticate as coming from the dialog peer or the SIP
Registrar/Proxy.
3.2.3. Example
Traffic management may be performed in the following way: The SBC
behaves as a B2BUA and inserts itself, or some other entity under the
operator's control, in the media path. In practice, the SBC modifies
the session descriptions carried in the SIP messages. As a result,
the SBC receives media from one user-agent and relays it to the other
user-agent and performs the identical operation with media traveling
in the reverse direction.
As mentioned in Section 3.2.1, codec restriction is a form of traffic
management. The SBC restricts the codec set negotiated in the offer/
answer exchange [5] between the user-agents. After modifying the
session descriptions, the SBC can check whether or not the media
stream corresponds to what was negotiated in the offer/answer
exchange. If it differs, the SBC has the ability to terminate the
media stream or take other appropriate (configured) actions (e.g.
raise an alarm).
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Consider the following example scenario: The SBC receives an INVITE
request from the outer network, which in this case is an access
network. The received SIP message contains the SDP session
descriptor shown in Figure 6.
v=0
o=owner 2890844526 2890842807 IN IP4 192.0.2.4
c=IN IP4 192.0.2.4
m=audio 49230 RTP/AVP 96 98
a=rtpmap:96 L8/8000
a=rtpmap:98 L16/16000/2
Figure 6: Request Prior to Media Management
In this example, the SBC performs the media traffic management
function by rewriting the 'm' line, and removing one 'a' line
according to some (external) policy. Figure 7 shows the session
description after the traffic management function.
v=0
o=owner 2890844526 2890842807 IN IP4 192.0.2.4
c=IN IP4 192.0.2.4
m=audio 49230 RTP/AVP 96
a=rtpmap:96 L8/8000
Figure 7: Request Body After Media Management
Media traffic management has a problem where the SBC needs to
understand the session description protocol and all extensions used
by the user-agents. This means that in order to use a new extension
(e.g., an extension to implement a new service) or a new session
description protocol, SBCs in the network may need to be upgraded in
conjunction with the endpoints. It is noteworthy that a similar
problem, but with header fields, applies to, for example, topology
hiding function, see Section 3.1. Certain extensions that do not
require active manipulation of the session descriptors to facilitate
traffic management will be able to be deployed without upgrading
existing SBCs, depending on the degree of transparency the SBC
implementation affords. In cases requiring an SBC modification to
support the new protocol features, the rate of service deployment may
be affected.
3.3. Fixing Capability Mismatches3.3.1. General Information and Requirements
SBCs fixing capability mismatches enable communications between user-
agents with different capabilities or extensions. For example, an
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SBC can enable a plain SIP [1] user agent to connect to a 3GPP
network, or enable a connection between user agents that support
different IP versions, different codecs, or that are in different
address realms. Operators have a requirement and a strong motivation
for performing capability mismatch fixing, so that they can provide
transparent communication across different domains. In some cases
different SIP extensions or methods to implement the same SIP
application (like monitoring session liveness, call history/diversion
etc) may also be interworked through the SBC.
3.3.2. Architectural Issues
SBCs that are fixing capability mismatches do it by insert a media
element to the media path using the procedures described in
Section 3.2. Therefore, these SBCs have the same concerns as SBCs
performing traffic management: the SBC may modify SIP messages
without consent from any of the user-agents. This may break end-to-
end security and application extensions negotiation.
The capability mismatch fixing is a fragile function in the long
term. The number of incompatibilities built into various network
elements is increasing the fragility and complexity over time. This
might lead to a situation where SBCs need to be able to handle a
large number of capability mismatches in parallel.
3.3.3. Example
Consider the following example scenario where the inner network is an
access network using IPv4 and the outer network is using IPv6. The
SBC receives an INVITE request with a session description from the
access network:
INVITE sip:callee@ipv6.domain.example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.2.4
Contact: sip:caller@u1.example.com
v=0
o=owner 2890844526 2890842807 IN IP4 192.0.2.4
c=IN IP4 192.0.2.4
m=audio 49230 RTP/AVP 96
a=rtpmap:96 L8/8000
Figure 8: Request Prior to Capabilities Match
Then the SBC performs a capability mismatch fixing function. In this
scenario the SBC inserts Record-Route and Via headers, and rewrites
the 'c' line from the sessions descriptor. Figure 9 shows the
request after the capability mismatch adjustment.
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INVITE sip:callee@ipv6.domain.com SIP/2.0
Record-Route: <sip:[2001:DB8::801:201:2ff:fe94:8e10];lr>
Via: SIP/2.0/UDP sip:[2001:DB8::801:201:2ff:fe94:8e10]
Via: SIP/2.0/UDP 192.0.2.4
Contact: sip:caller@u1.example.com
v=0
o=owner 2890844526 2890842807 IN IP4 192.0.2.4
c=IN IP6 2001:DB8::801:201:2ff:fe94:8e10
m=audio 49230 RTP/AVP 96
a=rtpmap:96 L8/8000
Figure 9: Request After Capability Match
This message is then sent by the SBC to the onward IPv6 network.
3.4. Maintaining SIP-related NAT Bindings3.4.1. General Information and Requirements
NAT traversal in this instance refers to the specific message
modifications required to assist a user-agent in maintaining SIP and
media connectivity when there is a NAT device located between a user-
agent and a proxy/registrar and, possibly, any other user-agent. The
primary purpose of NAT traversal function is to keep up a control
connection to user-agents behind NATs. This can, for example, be
achieved by generating periodic network traffic that keeps bindings
in NATs alive. SBCs' NAT traversal function is required in scenarios
where the NAT is outside the SBC (i.e., not in cases where SBC itself
acts as a NAT).
An SBC performing a NAT (Network Address Translator) traversal
function for a user agent behind a NAT sits between the user-agent
and the registrar of the domain. NATs are widely deployed in various
access networks today, so operators have a requirement to support it.
When the registrar receives a REGISTER request from the user-agent
and responds with a 200 (OK) response, the SBC modifies such a
response decreasing the validity of the registration (i.e., the
registration expires sooner). This forces the user-agent to send a
new REGISTER to refresh the registration sooner that it would have
done on receiving the original response from the registrar. The
REGISTER requests sent by the user-agent refresh the binding of the
NAT before the binding expires.
Note that the SBC does not need to relay all the REGISTER requests
received from the user-agent to the registrar. The SBC can generate
responses to REGISTER requests received before the registration is
about to expire at the registrar. Moreover, the SBC needs to
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deregister the user-agent if this fails to refresh its registration
in time, even if the registration at the registrar would still be
valid.
SBCs can also force traffic to go through a media relay for NAT
traversal purposes (more about media traffic management in
Section 3.2). A typical call has media streams to two directions.
Even though SBCs can force media streams from both directions to go
through a media relay, in some cases it is enough to relay only the
media from one direction (e.g., in a scenario where only the other
endpoint is behind a NAT).
3.4.2. Architectural Issues
This approach to NAT traversal does not work if end-to-end
confidentiality or integrity-protection mechanisms are used (e.g.,
S/MIME). The SBC would be seen as a MitM modifying the messages
between the user-agent and the registrar.
There is also a problem related to the method how SBCs choose the
value for the validity of a registration period. This value should
be as high as possible, but it still needs to be low enough to
maintain the NAT binding. Some SBCs do not have any deterministic
method for choosing a suitable value. However, SBCs can just use a
sub-optimal, relatively small value which usually works. An example
from such value is 15 seconds (see [9]).
NAT Traversal for media using SBCs poses few issues as well. For
example an SBC normally guesses the recipient's public IP address on
one of the media streams relayed by the SBC by snooping on the source
IP address of another media stream relayed by the same SBC. This
causes security and interoperability issues since the SBC can end up
associating wrong destination IP addresses on media streams it is
relaying. For example, an attacker may snoop on the local IP address
and ports used by the SBC for media relaying the streams and send a
few packets from a malicious IP address to these destinations. In
most cases, this can cause media streams in the opposite directions
to divert traffic to the attacker resulting in a successful MitM or
DoS attack. A similar example of an interop issue is caused when an
endpoint behind a NAT attempts to switch the IP address of the media
streams by using a re-INVITE. If any media packets are re-ordered or
delayed in the network, they can cause the SBC to block the switch
from happening even if the re-INVITE successfully goes through.
3.4.3. Example
Consider the following example scenario: The SBC resides between the
UA and Registrar. Previously the UA has sent a REGISTER request to
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Registrar, and the SBC receives the registration response shown in
Figure 10.
SIP/2.0 200 OK
From: Bob <sip:bob@biloxi.example.com>;tag=a73kszlfl
To: Bob <sip:bob@biloxi.example.com>;tag=34095828jh
CSeq: 1 REGISTER
Contact: <sips:bob@client.biloxi.example.com>;expires=3600
Figure 10: Response Prior to NAT Maintenance Function
When performing the NAT traversal function, the SBC may re-write the
expiry time to coax the UA to re-register prior to the intermediating
NAT deciding to close the pinhole. Figure 11 shows a possible
modification of the response from Figure 10.
SIP/2.0 200 OK
From: Bob <sip:bob@biloxi.example.com>;tag=a73kszlfl
To: Bob <sip:bob@biloxi.example.com>;tag=34095828jh
CSeq: 1 REGISTER
Contact: <sips:bob@client.biloxi.example.com>;expires=60
Figure 11: Manipulated Response for NAT Traversal
Naturally also other measures could be taken in order to enable the
NAT traversal (e.g., non-SIP keepalive messages), but this example
illustrates only one mechanism for preserving the SIP-related NAT
bindings.
3.5. Access Control3.5.1. General Information and Requirements
Network operators may wish to control what kind of signaling and
media traffic their network carries. There is strong motivation and
a requirement to do access control on the edge of an operator's
network. Access control can be based on, for example, link-layer
identifiers, IP addresses or SIP identities.
This function can be implemented by protecting the inner network with
firewalls and configuring them so that they only accept SIP traffic
from the SBC. This way, all the SIP traffic entering the inner
network needs to be routed though the SBC, which only routes messages
from authorized parties or traffic that meets a specific policy that
is expressed in the SBC administratively.
Access control can be applied either only to the signaling, or to
both the signaling and media. If it is applied only to the
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signaling, then the SBC might behave as a proxy server. If access
control is applied to both the signaling and media, then the SBC
behaves in a similar manner as explained in Section 3.2. A key part
of media-layer access control is that only media for authorized
sessions is allowed to pass through the SBC and/or associated media
relay devices.
Operators implement some functionalities, like NAT traversal for
example, in an SBC instead of other elements in the inner network for
several reasons: (i) preventing packets from unregistered users to
prevent chances of DoS attack, (ii) prioritization and/or re-routing
of traffic (based on user or service, like E911) as it enters the
network, and (iii) performing a load balancing function or reducing
the load on other network equipment.
In environments where there is limited bandwidth on the access links,
the SBC can compute the potential bandwidth usage by examining the
codecs present in SDP offers and answers. With this information, the
SBC can reject sessions before the available bandwidth is exhausted
to allow existing sessions to maintain acceptable quality of service.
Otherwise, the link could become over-subscribed and all sessions
would experience a deterioration in quality of service. SBCs may
contact a policy server to determine whether sufficient bandwidth is
available on a per-session basis.
3.5.2. Architectural Issues
Since the SBC needs to handle all SIP messages, this function has
scalability implications. In addition, the SBC is a single point of
failure from an architectural point of view. Although, in practice,
many current SBCs have the capability to support redundant
configuration, which prevents the loss of calls and/or sessions in
the event of a failure on a single node.
If access control is performed only on behalf of signaling, then the
SBC is compatible with general SIP architectural principles, but if
it is performed for signaling and for media, then there are similar
problems as described in Section 3.2.2.
3.5.3. Example
Figure 12 shows a callflow where the SBC is providing both signaling
and media access control (ACKs omitted for brevity).
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caller SBC callee
| | |
| Identify the caller | |
|<- - - - - - - - - - - >| |
| | |
| INVITE + SDP | |
|----------------------->| |
| [Modify the SDP] |
| | INVITE + modified SDP |
| |----------------------->|
| | |
| | 200 OK + SDP |
| |<-----------------------|
| [Modify the SDP] |
| | |
| 200 OK + modified SDP | |
|<-----------------------| |
| | |
| Media [Media inspection] Media |
|<======================>|<======================>|
| | |
Figure 12: Example Access Callflow
In this scenario, the SBC first identifies the caller, so it can
determine whether or not to give signaling access for the caller.
This might be achieved using information gathered during
registration, or by other means. Some SBCs may rely on the proxy to
authenticate the user-agent placing the call. After identification,
the SBC modifies the session descriptors in INVITE and 200 OK
messages in a way so that the media is going to flow through the SBC
itself. When the media starts flowing, the SBC can inspect whether
the callee and caller use the codec(s) that they had previously
agreed on.
3.6. Protocol Repair3.6.1. General Information and Requirements
SBCs are also used to repair protocol messages generated by not-
fully-standard compliant or badly implemented clients. Operators may
wish to support protocol repair, if they want to support as many
clients as possible. It is noteworthy, that this function affects
only the signaling component of an SBC, and that the protocol repair
function is not the same as protocol conversion (i.e., making
translation between two completely different protocols).
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Internet-Draft Requirements from SIP SBC Deployments February 20103.6.2. Architectural Issues
In many cases, doing protocol repair for SIP header fields can be
seen as being compatible with SIP architectural principles, and it
does not violate the end-to-end model of SIP. The SBC repairing
protocol messages behaves as a proxy server that is liberal in what
it accepts and strict in what it sends.
However, protocol repair may break security mechanism that do
cryptographical computations on SIP header values. Attempting
protocol repair for SIP message bodies (SDP) is incompatible with
Authenticated Identity Management [4] and end-to-end security
mechanisms such as S/MIME.
A similar problem related to increasing complexity, as explained in
Section 3.3.2, also affects protocol repair function.
3.6.3. Examples
The SBC can, for example, receive an INVITE message from a relatively
new SIP UA as illustrated in Figure 13.
INVITE sip:callee@sbchost.example.com
Via: SIP/2.0/UDP u1.example.com:5060;lr
From: Caller <sip:caller@one.example.com>
To: Callee <sip:callee@two.example.com>
Call-ID: 18293281@u1.example.com
CSeq: 1 INVITE
Contact: sip:caller@u1.example.com
Figure 13: Request from a relatively new client
If the SBC does protocol repair, it can re-write the 'lr' parameter
on the Via header field into the form 'lr=true', in order to support
some older, badly implemented SIP stacks. It could also remove
excess white spaces to make the SIP message more human readable.
3.7. Media Encryption3.7.1. General Information and Requirements
SBCs are used to perform media encryption / decryption at the edge of
the network. This is the case when media encryption (e.g., Secure
Real-time Transport Protocol (SRTP)) is used only on the access
network (outer network) side and the media is carried unencrypted in
the inner network. Some operators provide the ability to do legal
interception while still giving their customers the ability to
encrypt media in the access network. One possible way to do this is
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path. The same happens in the second SBC. Then the UAS replies with
a 200 OK response and the SBCs inject themselves in the returning
media path. After signaling the media starts flowing, and both SBCs
are performing media encryption and decryption.
4. Derived Requirements for Future SIP Standardization Work
Some of the functions listed in this document are more SIP-unfriendly
than others. This list of requirements is derived from the functions
that break the principles of SIP in one way or another when performed
by SBCs that do not have the users' consent. The derived
requirements are:
Req-1: There should be a SIP-friendly way to hide network topology
information. Currently this is done by stripping and
replacing header fields, which is against the principles of
SIP on behalf of some header fields (see Section 3.1).
Req-2: There should be a SIP-friendly way to direct media traffic
through intermediaries. Currently this is done by modifying
session descriptors, which is against the principles of SIP
(see Section 3.2, Section 3.4, Section 3.5, and Section 3.7).
Req-3: There should be a SIP-friendly way to fix capability
mismatches in SIP messages. This requirement is harder to
fulfill on complex mismatch cases, like the 3GPP/SIP [1]
network mismatch. Currently this is done by modifying SIP
messages, which may violate end-to-end security (see
Section 3.3 and Section 3.6), on behalf of some header
fields.
Req-1 and Req-3 do not have an existing, standardized solution today.
There is ongoing work in the IETF for addressing Req-2, such as SIP
session policies [10], Traversal Using Relays around NAT (TURN) [11],
and Interactive Connectivity Establishment (ICE) [12]. Nonetheless,
future work is needed in order to develop solutions to these
requirements.
5. Security Considerations
Many of the functions this document describes have important security
and privacy implications. One major security problem is that many
functions implemented by SBCs (e.g., topology hiding and media
traffic management) modify SIP messages and their bodies without the
user agents' consent. The result is that the user agents may
interpret the actions taken by an SBC as a MitM attack. SBCs modify
SIP messages because it allows them to, for example, protect elements
in the inner network from direct attacks.
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SBCs that place themselves (or another entity) on the media path can
be used to eavesdrop on conversations. Since, often, user agents
cannot distinguish between the actions of an attacker and those of an
SBC, users cannot know whether they are being eavesdropped or an SBC
on the path is performing some other function. SBCs place themselves
on the media path because it allows them to, for example, perform
legal interception.
On a general level SBCs prevent the use of end-to-end authentication.
This is because SBCs need to be able to perform actions that look
like MitM attacks, and in order for user agents to communicate, they
must allow those type of attacks. It other words, user agents can
not use end-to-end security. This is especially harmful because also
other network element, besides SBCs, are then able to do similar
attacks. However, on some cases, user agents can establish encrypted
media connections between each other. One example is a scenario
where SBC is used for enabling media monitoring, but not for
interception.
An SBC is a single point of failure from the architectural point of
view. This makes it an attractive target for DoS attacks. The fact
that some functions of SBCs require those SBCs to maintain session-
specific information makes the situation even worse. If the SBC
crashes (or is brought down by an attacker), ongoing sessions
experience undetermined behavior.
If the IETF decides to develop standard mechanisms to address the
requirements presented in Section 4, the security and privacy-related
aspects of those mechanisms will, of course, need to be taken into
consideration.
6. IANA Considerations
This document has no IANA considerations.
7. Acknowledgements
The ad-hoc meeting about SBCs, held on Nov 9th 2004 at Washington DC
during the 61st IETF meeting, provided valuable input to this
document. Authors would also like to thank Sridhar Ramachandran,
Gaurav Kulshreshtha, and Rakendu Devdhar. Reviewers Spencer Dawkins
and Francois Audet also deserve special thanks.
8. ReferencesHautakorpi, et al. Expires August 22, 2010 [Page 23]